Production of protective coating on aluminum and noncuprous alloys thereof



Patented Dec. 29, 1936,

PRODUCTION OF PROTECTIVE COATING 0 *UMINUM AND NONCUPROUS AL-v N A...LOYS THEREOF PATE T OFFICE;

Delaware No Drawing. Original application June 18, 1935, Serial No.27,208. Divided and this application June 18, 1935, Britain July 13,1934 Serial No. 27,209. In Great 11 Claims. (c1. 148 -6) Thisapplication is a division of application Serial No. 27,208, filedconcurrently herewith.

The present invention relates to a process of and composition for use inproducing protective or paint-holding coatings on aluminum andnon-cuprous alloys thereof, and has for its gen-.

eral object to improve the protective or paintholding properties of suchcoatings.

It is already known that protective coatings can be formed onnon-cuprous aluminum-containing articles by treatment with a solutioncontaining an alkali metal carbonate and an alkali metal chromate.According to the invention, denser and harder films are produced bymaking certain additions to solutions which essentially comprise suchcarbonates and chromates. These additions comprise broadly the neutralsalts of an alkali metal and those carbonates of a metalelectro-positive with respect to aluminum, including basic carbonates,which do not react with any of the other constituents of the solution.Any one or more of these sub stances may be employed in the solution,the first-mentioned group of substances, namely, the neutral salts of analkali metal, being claimed separately herein.

Examples of suitable neutral salts of alkali metals are sodiummonohydrogen phosphate, sodium chloride and sodium nitrate, whileexamples of suitable metallic carbonates are chromium carbonate, and thecarbonates of nickel,

aluminum, cobalt, manganese and titanium. Iron carbonate reacts with thesodium carbonate in the solution and is therefore not sui able. Althoughthe invention is not to be held limited in any way by any particulartheory to account for the effects observed, it is believed that thefunction of the neutral alkali metal salt is to buffer the solution andthus prevent the hydrogen ion concentration from. fiuctuat ingexcessively, keeping it within narrow limits and thereby insuring moreuniform results.

These salts do not enter into the composition,

of the final coating, and potassium' or sodium salts maybe usedindiscriminately. The me= tallic carbonates used, on the other hand, doaiiect the composition of the final coating obtained. The alkali metalsalt is added to the solution in small amount, preferably such that itstotal content in the solution isbetween 0.25 and0.5%, and it should notexceed while the metallic carbonate is preferablyadded in amounts suchthat the dry material or powder used to form the solution contains from3 to 8% of metallic carbonate and preferably 5%,

the solution then being formed by dissolving '1 .5? parts ofthe powderin 100 parts of water. It is difiicult to ascertain the amount ofmetallic carbonate actually present in solution, because in practicesuch substance is found to precipitate as sludge.

Particularly good results are obtained when chromium carbonate isemployed as this compound serves to give an especially hard and densefilm and also helps to maintain the soluble chromium content of thesolution.

The solutions according to the present invention are best worked at atemperature between and 0., but it is possible to reduce the workingtemperature by addition of caustic soda and by increasingtheconcentration of the constituents.

In order that the invention may be clearly understood and readilycarried into effect, some examples will now be given of its application.

Example I Parts by weight Sodium carbonate--- 72.7.- Sod-ium chromate24.2

Sodium monohydrogen phosphate 3.0

After thistreatment the article is rinsed in not water. It is then readyfor use for the reception of paints, lacquers, enamels and the like, oralternatively the treated surface maybe further protected by treatmentwith a such as oline,

Example If The conditions are the same as in Example I, but the bath isprepared by dissolving in 100 parts of water 7.8 parts of a powderhaving the following composition:

Parts by weight Sodium carbonate 69.4 Sodium ChI0mat 23.6

Chromium carbonate 7 Example III The temperature of treatment is reducedto 70 C. by the use of abath prepared by dissolving substance in 100parts of water 11.5 parts of a powder having the following composition:

I, 7 Parts by weight Sodium carbonate 67.9 Sodium chromate -l. 22.3Sodium hydroxide 4.9 Chromium carbonate s 4.9

The treatment is otherwise similar to that described in Example I.

Broadly speaking thepowder used to make up the bath initially shouldhave a composition such that 100 parts by weight of the powder contain:

, Parts Anhydrous sodium carbonate 40 to 80 Anhydrous sodium chromate52to 15 Metallic carbonate 3 to 8 A particular example of such a powderwhich is found to give good results is the following;

I Parts Anhydrous sodium carbonate 71.4 Anhydrous sodium chromate 21.4-Chromium carbonate 7.2

The bath is prepared from the powder specified by dissolving 5 to partsin 100 parts of water; Since the chromate is used up at a greater ratethan the carbonate during the working of the bath, the replenishingpowder or dry material to be added for replenishing the bath needs tocontain'a larger proportion of chromate than the initial powder.Accordingly the composition of the replenishing powder is as follows:

, Parts Anhydrous sodium carbonate s 35 to 65 Anhydrous sodium chromate5'7 to Metallic carbonate 3to 8 A preferred composition of thereplenishing powder is as follows:

, v Parts Anhydrous sodium carbonate 57.6 Anhydrous sodium, chromate35.2 Chromium carbonate 7.2

ing paint or may be dyed more uniformly than films produced bybathswithout additions according to the invention.

It is desired to have it understood that the process which has beendescribed is subject to various modifications without departing from thespirit of the invention. Thus, while the invention is particularlyapplicable to coating by immersionin baths, the constituents may be madeup into a paste which may be sprayed or brushed onto the surface of thearticle to be treated.

What is claimed is:

l. The process of producing protective or paintholding coatings onaluminum and non-cuprous alloys thereof, which comprises treating thenon- I cuprous aluminum-containing article to be coated withan aqueouscomposition containing an alkali metal carbonate, an alkali metalchromate, and

a neutral salt of an alkali metal, the concentration .of the neutralalkali metal salt being maintained at a value not exceeding 1%.

2. The process of producing protective or paintholding coatings onaluminum and non-cuprous alloys thereof {which comprises treating thenoncuprous aluminum-containing article to be coated with an aqueouscomposition containing an alkali metal carbonate, an alkali metalchromate,

and sodium monohydrogen phosphate, the concentration of sodiummonohydrogen phosphate being maintained at a value not exceeding 1%.

3. The process of producing protective or paintholding coatings onaluminum and non-cuprous alloys thereof, which comprises treating thenoncuprous aluminum-containing article to be coated with an aqueouscomposition containing an alkali metal carbonate, an alkali metalchromate, and a neutral salt of an alkali metal, the concentration ofthe neutral alkali metal salt being maintained between 0.25 and 0.5%.

4. The process of producing protective or paintholding coatings onaluminum and non-cuprous alloys thereof, which comprises treating thenoncuprous aluminum-containing article to be coated with an aqueouscomposition containing an alkali metal carbonate, an alkali metalchromate, and sodium monohydrogen phosphate, the concentration of thesodium monohydrogen phosphate being maintained between 0.25 and 0.5%.

5. A bath for use in the production of protective or paint-holdingcoatings on aluminum and non-cuprous alloys thereof, which comprises inaqueous solution an alkali metal carbonate, an alkali metal chromate,and a {neutral salt of an alkali metal in quantity corresponding to aconcentration not exceeding 1%.

6. A bath for use in the production of protective or paint-holdingcoatings on aluminum and non-cuprous alloys thereof, which comprises inaqueous solution an alkali metal carbonate, an alkali metal chromate,and sodium monohydrogen phosphate in quantity corresponding to aconcentration not exceeding 1%.

7. A bath for use in the production of protective or paint holdingcoatings on aluminum and non-cuprous alloys thereof, which comprises inaqueous solution an alkali metal carbonate, an alkali metal chromate,and a neutral salt of an alkali metal in quantity corresponding to aconcentration of from 0.25 to 0.5%.

8. A bath for use in the production of protective or paint-holdingcoatings on aluminum and non-cuprous alloys thereof, which comprises inaqueous solution an'alkali metal carbonate, an

' alkali metal chromate, and sodium monohydrogen phosphate in quantitycorresponding to a concentration of from 0.25 to 0.5%.

9. The process of producing protective or paintso i holding coatings onaluminum and non-cuprous alloys thereof, which comprises treating thenoncuprous aluminum-containing article to be coated with an aqueouscomposition containing an alkali metal carbonate, an alkali metalchromate, so-

. dium hydroxide, and a'neutral salt of an alkali metal, theconcentration of the neutral alkali metal salt being maintained at avaluenot exceeding 1%.

'10. The process of producing protective or paint-holding coatings onaluminum and noncuprous alloys thereof, which comprises treating the,non-cuprous aluminum-containing article to be coated with an aqueouscomposition containing an alkali metal carbonate, an alkali metalchromate, and sodium'chloride, the concentration of sodium chloridebeing maintained at a cuprous the non alloys thereof, which comprisestreating -cuprous aluminum-containing article to be coated with anaqueous composition containing an alkali metal carbonate, an alkalimetal chromate, and sodium nitrate, the concentration of sodium nitratebeing maintained at a value not exceeding 1%.

EDWIN CECIL FREDERICK KING.

